Chromospheric and transition-region emission from young solar-type stars in clusters, kinematic groups, and the field

Details Chromospheric and transition-region emission from young solar-type stars in clusters, kinematic groups, and the field Chromospheric and transition-region emission from young solar-type stars in clusters, kinematic groups, and the field

Apr 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987aj.....93..920s&link_type=abstract

Astronomical Journal (ISSN 0004-6256), vol. 93, April 1987, p. 920-937.

Astronomy and Astrophysics

Astronomy

55

Eclipsing Binary Stars, Emission Spectra, Star Clusters, Stellar Atmospheres, Stellar Evolution, Stellar Spectrophotometry, Chromosphere, Iue, Solar Neighborhood, Spaceborne Astronomy, Stellar Motions, Transition Points

Scientific paper

We have examined the strength of chromospheric and transition-region emission in solar-type stars that have been suggested to be members of the Ursa Major Group and Hyades Group (originally selected on kinematic grounds), as well as stars in the Hyades Cluster and other young stars of the solar neighborhood. We find that indices of the Ca II (R'HK) and Mg II (Rhk) emissions are homogeneous among stars of the Hyades Cluster. This corroborates the usefulness of these indices as age indicators for solar-type stars, and also indicates that a phenomenon like the Sun's Maunder minimum is rare among young stars. The Rhk and R'HK indices are not as homogeneous for the Ursa Major Group ("UMaG") as for the Hyades, unless one is very restrictive about membership, to the point where stars that are valid kinematic members are excluded. About 40% of the stars suggested to be UMaG members are rejected because their chromospheric emission is too weak. The remaining probable and possible UMaG members cluster tightly in velocity space, particularly in the V coordinate, as expected for a true kinematic group. Once attention is restricted to this diminished volume of velocity space, one finds few contaminating stars. For the Hyades Group, three of the four candidates are probably associated with the cluster. For the remaining stars of the field, there are no kinematic groups that are as prominent as UMaG. A few stars have the appearance of being as young as stars in the Pleiades cluster, but they cannot be convincingly associated with the cluster on the basis of their kinematics. The difference in mean chromospheric emission levels for UMaG and the Hyades is less than expected from their age difference, given the emission-age relation seen in older stars. The most likely explanation is that the chromospheric emission is nearly level for very young stars, and starts on its power law decline (R'HK ∝ t-1/2, t = age) near the age of the Hyades or UMaG.

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